>Even when used without the extender infinity is not exactly at the
>'mechanical end' of the lens. Perhaps 1 to 2 mm from the end to be tack
>sharp.
>
>Is that something to worry?
The exotic glasses used in apochromatic telephotos can expand and
contract (at least this is how it was explained to me) causing infinity
focus to vary according to temperature. It is nothing to worry about and
usually in fast telephoto lenses you'll notice that there is some focus
travel past the infinity mark possible to account for such variations.